A turbomachine turbine comprises a plurality of stages, each made up of a stationary stator grid or row and a moving rotor grid or row. Each row is made up of an annular row of airfoils that are regularly distributed around the longitudinal axis of the turbine. The airfoils of a rotor row are carried by a disk connected to the shaft of the turbine, and the airfoils of a stator row are connected to an outer casing of the turbine.
The airfoils of the rows extend substantially across the entire radial extent of the flow section in which hot gas flows through the turbine. The hot gas leaves the combustion chamber of the turbomachine and flows through the turbine, thereby delivering energy to the rotor rows that drive the turbine shaft in rotation.
In order to improve the performance of the turbine, it is known to implement multistage aerodynamic coupling between two consecutive rotor rows separated from each other by a stator row, or between two consecutive stator rows separated from each other by a rotor row.
Multistage aerodynamic coupling consists in selecting two consecutive rows of the same type (i.e. two rotor rows or two stator rows) and having the same number of airfoils or having a number of airfoils that is a multiple of the number of airfoils of the same type situated upstream, and in angularly positioning the downstream row relative to the upstream row in such a manner that the wakes formed from the trailing edges of the airfoils of the upstream row impact within a certain tolerance on the leading edges of the airfoils of the downstream row.
Methods have already been proposed for designing a turbine so as to achieve such coupling. Nevertheless, those methods make use of complex calculations that are very expensive in terms of time, thereby making them incompatible with conventional design deadlines. Furthermore, those methods do not always take account of multistage aerodynamic coupling over the full height of the turbine flow section, i.e. over the entire radial extent of the airfoils of the rows. Finally, presently available methods are applied only on a pair of rows of the same type, i.e. on one-and-a-half stages of the turbine (stator row/rotor row/stator row or rotor row/stator row/rotor row), and they therefore need to be repeated several times over in order to design all of the stages of the turbine. This approach presents a major drawback: the pairs of rows are positioned angularly independently of other pairs of rows, whereas each row ought to depend on the angular position of the row situated upstream and ought to influence the angular position of the row situated downstream, while taking account of the full height of the turbine flow section. Unless recourse is had to a lengthy iterative process that is expensive in terms of computation time, the drawback leads to a turbine configuration that is not optimized for multistage aerodynamic coupling.